1. Field of the Invention
The present invention relates to data storage devices of the type that store data on the surfaces of magnetic disk media. More particularly, the invention relates to a system and method for writing servo patterns within the servo regions of data storage media adapted for use in such apparatus.
2. Description of the Prior Art
By way of background, magnetic disk-based storage systems, such as the disk drive 2 of FIG. 1, use servo control systems to maintain highly accurate positional relationships between the transducers 4 that read and write data, and the disk surfaces 6 where the data is stored in a plurality of tracks 8. The servo control system periodically monitors the position of whichever transducer 4 has been selected for active data read/write operations, and makes necessary positional adjustments by providing control signals to a motor that pivots a transducer-carrying actuator 10.
A conventional disk drive servo control system includes servo control logic (within the drive controller 12) that processes servo information read from the disk surface 6 into positioning information that is used to produce the required control signals. The servo information is commonly recorded in servo sectors that are interspersed with data regions of the tracks 8 that store customer data. This is sometimes referred to as sector servo recording, and is performed by the disk drive manufacturer prior to final drive assembly in an operation known as servo writing. Because the servo sectors are generally placed at the same circumferential locations on each track, they tend to be aligned in servo regions that extend in a cross-track direction. FIG. 1 shows three such regions 14. Typically, there are multiple (e.g. 80-90 or more) servo regions per disk surface, with each servo region being separated by a customer data region.
As can be seen in FIG. 2a, the servo sectors that comprise the servo regions of a disk drive contain various information fields, including a write recovery/leading timing gap, an automatic gain control field (AGC), a servo timing mark (STM), a track-ID field (TID), a position error signal field (PES), and a trailing timing gap. The timing gaps serve to separate the servo sectors from the surrounding data regions. The AGC field is used to set the servo gain and clocking in the servo control logic to allow the reading of the subsequent servo information. The STM provides bit alignment for reading the subsequent servo information. The TID field typically records the track number (coarse position) and other digital information, such as sector number, index and head number. The PES field is designed to provide fine track positioning information.
As shown in FIG. 2b, a conventional PES field contains a pattern of analog servo bursts that are used to determine transducer position relative to a track centerline. There are typically four servo bursts in each servo pattern that are arranged in what is commonly referred to as a “quad burst pattern.” The servo bursts, which are labeled, “A,” “B,” “C” and “D” in FIG. 2b, are offset from each other relative to the track centerline, and are also spaced in the circumferential trackwise direction. During fine track servo positioning, read back signals produced by the servo bursts are demodulated and compared to generate a position error sensing (PES) signal. Typically, there are two PES signal components, namely, a primary signal component called “P” and a quadrature signal component called “Q,” each being 90° out of phase with the other.
With respect to the readback signal generated when the transducer is over the A, B, C or D servo bursts, the primary P signal component is typically formed by calculating the difference in signal strength between the A and B read back signals according to the relationship P=A−B. Similarly, the quadrature Q signal component is typically formed by calculating the difference in signal strength between the C and D read back signals according to the relationship Q=C−D. The point at which the P and Q signal components are zero (i.e., the read back signals from each burst are of equal strength) is referred to as the “burst centerline” because it represents the physical centerline between the two bursts that comprise the signal. In other words, P=0 is the centerline between the A and B bursts, and Q=0 is the centerline between the C and D bursts. As exemplified by FIG. 2b, the A and B bursts are typically recorded so that their burst centerline corresponds to a track centerline. Similarly, the C and D bursts are typically recorded so that their burst centerline is halfway between the centerlines of two adjacent tracks (i.e., at ½ track pitch offsets).
The PES signal is generated by alternating between the P and Q components according to which one is more linear (based on transducer position in the trackwise direction) to provide a PES signal with the greatest linearity. Thus, when a transducer is at or near a track centerline, the P signal component (A−B) will be used. When the transducer is at or near a ½ track offset position, the Q signal component (C−D) will be used. Generally speaking, the cutoff point for transitioning between the P and Q signal components is when the transducer is half way between these two positions, which corresponds to the transducer being offset ¼ track on either side of a track centerline.
As indicated, the above mentioned servo patterns are recorded during disk drive manufacture in a process known as servo writing. More particularly, each transducer is used to write its own servo information that will later be used to position the transducer during read/write operations. Before describing this process in more detail, it will be helpful to review the construction of a conventional disk drive transducer. As shown by FIGS. 3-6, a typical transducer T includes an inductive write head element W for writing data and a magneto-resistive read head element R for reading data. The write head W includes a pair of soft ferromagnetic film layers P1 and P2 that extend from a back gap area BG to an ABS (Air Bearing Surface) that is adapted to magnetically interact with the disk's recording surface. There, the P1 and P2 layers respectively form pole tips PT1 and PT2. The pole tips are separated by an insulative gap layer G3 that defines the head's write gap. An electromagnetic coil structure C is sandwiched between the P1 and P2 layers to define the yoke portion of the write head W. The yoke extends from the back gap BG to the pole tips PT1 and PT2. Insulative layers I1, I2 and I3 electrically insulate the coil structure C from the P1 and P2 layers.
During write operations, electrical current passing through a pair of electrical leads E1 and E2 to the coil C generates a magnetic field that induces a magnetic flux in the P1 and P2 layers. As shown in FIG. 5, this magnetic flux propagates from the yoke to the pole tips PT1 and PT2, where it fringes across the G3 gap layer. This causes magnetic domains to be formed on the underlying recording surface. The orientation of each recorded magnetic domain is dependent on the magnetization direction of the pole tips PT1 and PT2, which in turn is determined by the direction of the electrical current passing through the coil C. Reversing the coil's electrical current reverses the magnetization direction of the pole tips PT1 and PT2, and consequently reverses the orientation of the next recorded magnetic domain. This magnetization reversal process is used to encode data on the recording surface.
The read head R lies between a pair of insulative G1 and G2 gap layers at the ABS. It is typically formed as a layered structure having magneto-resistive properties. An electrical sense current passing through the read head R will thus be modulated by the magnetic domains formed by the write head W, which induce alterations of the magnetic and electrical properties of the read head.
FIG. 6 depicts the transducer T from the vantage point of the disk surface so as to illustrate the track width dimensions of the write head W and the read head T. The write head track width (Tw) determines the width of the magnetic domains that are written on the disk surface. The read head track width (Tr) is optimized to allow the read head R to sense the magnetic domains written by the write head W with good signal-to-noise ratio. Generally speaking, the write head track width Tw is designed to be approximately 80% of the track pitch Tp between adjacent track centerlines.
A common way to record servo patterns is to form each servo burst using several passes of the transducer write head, with the write head being stepped at sub-track pitch increments (such as one-half the track pitch or less) for each pass. According to this approach, a first “burst stripe” of a servo burst is written by the write head, and then the transducer is stepped by ½ track (or less) to write the next burst stripe of the servo burst. The transducer is then stepped another ½ track increment (or less) and the bottom radial (trackwise) edge of the servo burst is erased in a process known as trimming. Servo patterns produced in this manner are commonly referred to as “seamed trimmed” patterns.
In the interest of controlling TMR, various seamless untrimmed servo patterns have been proposed. In modem high track density disk drives, this typically means using what is known as a “seamless untrimmed” servo burst pattern in which each servo burst is written in a single pass at the width of the write head. When implemented in a half-track pattern in which adjacent servo bursts are radially offset from each other by ½ track, this can reduce servo writer-induced TMR up to two times.
Seamless untrimmed servo patterns are thus generally preferred over seamed trimmed servo patterns. However, this assumes that all transducers are alike, when in fact there can be significant tolerances producing head geometries that are not compatible with seamless untrimmed servo patterns. Transducers having read and write heads that are both relatively narrow relative to track pitch (Tp) are particularly problematic when used with seamless untrimmed servo patterns. In that case, the narrow write head will write narrow servo bursts, which produces excessive radial gaps between adjacent servo bursts. If the gaps are wide enough relative to the narrow read head, the read head could miss both servo bursts when positioned over a burst pair centerline such that no useful PES signal is obtained. Transducers having wide write heads and narrow read heads can also experience problems. In this case, the wide write head will write wide servo bursts. If the read head is narrow enough relative to the wide servo bursts, the read head could pass over a servo burst when positioned between burst pair centerlines without seeing an edge of the burst. This saturates the read head such that no useful PES signal is obtained. Both of the foregoing conditions may be referred to a “flat-topping” because the slope of the P and Q signals as a function of read head radial position becomes zero or flat. When flat-topping occurs, any change in read head position will produce no corresponding change in PES signal, such that accurate servo positioning becomes impossible.
FIG. 7 is illustrative. It shows four servo bursts A, B, C, and D servo bursts that are each one track pitch wide (Tw=Tp), and a read head that is also one track pitch wide (Tr=Tp). The graph at the lower portion of FIG. 7 shows the phase differentiated PES signal components based on P=A−B and Q=C−D. The superimposed dark lines in the graph show the point at which the P and Q signal components are used for read head positioning as the read head move radially across the disk. In particular, the P=A−B signal component is used when the read head is centered near the track centerlines (e.g., TK 0, TK 1, TK 2, TK 3, etc.) and the Q=C−D signal component is used when the read head is centered between tracks. As described above, the transition point between the P and Q signal components is when the read head is offset ¼ track from a track centerline. Note that the PES signals are relatively linear between the ¼ track offset locations, such that a given change in signal strength corresponds to a readily determinable change in transducer position.
It can be seen from FIG. 7 that if the read and write head track widths Tr and Tw are small enough, that there will be radial positions at which the read head could become lost between servo bursts separated by wide gaps. Similarly, if the read head track width Tr is small in comparison to the write head track width Tw, there will be radial positions at which the read head becomes saturated by a servo burst.
The foregoing head incompatibility problems have increased in recent years as a result of reductions in track pitch, which have outpaced the ability to reduce manufacturing tolerances. A larger number of head geometries are thus subject to higher PES nonlinearities when used with seamless untrimmed servo patterns. Because the prevailing wisdom is to select a single servo pattern for use with all heads, it has become more difficult to select a nominal servo pattern design point that will work for all heads.
Although linearization processes have been proposed to address the PES nonlinearity problems, the flat-top problem cannot be cured by these methods. The reality is that when untrimmed servo burst patterns are used, there will always be certain transducers produced during a given manufacturing run that are simply unsuitable for use. A solution to this problem is therefore needed so that transducers which would have previously been rejected may now be considered for use in disk drive products.